What were “burghal defenses” designed to stop?

The story behind the answer

Burghal defenses were a planned network of fortified towns (burhs) created in late 9th–early 10th century England, especially under Alfred the Great, to counter fast-moving Viking hit-and-run forces. By giving communities a nearby strongpoint for refuge and a place to muster the local militia (fyrd), the system denied attackers easy plunder and safe withdrawal, making quick strikes risky and unprofitable.

Set out so that no settlement was far from a burh, the network is recorded in the Burghal Hidage, which even calculated garrison sizes from wall length and local tax units (hides). These sites featured earth ramparts, deep ditches, timber palisades, and sometimes fortified bridges that blocked river routes. Over time, many burhs evolved into enduring market and administrative centers—foundations of later English towns.